Preserve nested Platform write targets in Babel preset

[OskarEichler]

Summary:

The React Native Babel preset replaces Platform.OS with a string literal even when the member expression is nested inside a destructuring assignment target. Direct and nested for...in / for...of targets are affected as well; direct loop targets can make Babel abort because a string literal is not a valid assignment target.

Walk the member expression's ancestors and preserve it whenever the containing expression is the left side of an assignment or loop, or the argument of an update/delete operation. This prevents invalid generated syntax and keeps write semantics intact. Metro has a parallel transform, so companion Metro PR #1890 applies the same guard there.

Changelog:

[GENERAL] [FIXED] - Preserve nested Platform.OS write targets in the Babel preset.

Test Plan:

• Added regressions for array/object destructuring assignments and direct/nested for...in and for...of targets.
• Exact baseline rewrites destructuring targets and fails Babel validation for a direct loop target; the fixed suite preserves every write target.
• Full preset Jest passes: 4/4 suites, 112/112 tests, 16 snapshots.
• Fresh Flow check reports 0 errors.
• Targeted no-ignore ESLint, Prettier, and git diff --check pass.

No public API or intended read-transform behavior changes.

[OskarEichler]

A second pass found that the ancestor walk treated every expression nested anywhere under an assignment LHS as a write. That incorrectly prevented inlining ordinary reads such as target[Platform.OS] = value and computed keys in destructuring targets. The walk now continues only through actual assignment-pattern containers and stops at regular expressions. Added regressions cover direct computed members, nested array targets, and computed object keys. The focused suite passes (53 tests / 11 snapshots), with targeted ESLint and Prettier checks passing.

[robhogan]

Thanks!